The present invention relates to direct current dynamoelectric machines and, more particularly, to a machine with circumferentially segmented magnets.
The present invention is closely related to and is an improvement on the subject matter of copending application Ser. No. 891,564 by Mole et al, filed Mar. 29, 1978 and assigned to the present assignee, now U.S. Pat. No. 4,185,216, issued Jan. 22, 1980. The full disclosure of said copending application is herein incorporated by reference.
In the copending application there is described a dynamoelectric machine in which magnetic fields are developed by field windings placed in longitudinally running recesses in a cylindrical stator and energized so as to provide radial magnetic fields in polar regions between the field windings. Stator conductors are placed in slots in the polar regions. The rotor carries longitudinal conductors on its surface positioned to cut the radial magnetic fluxes, and current collecting means are provided at both ends of the machine to make electrical contact with both ends of the rotor conductors. The current collecting means is connected to the stator conductors to complete the electrical circuit of the machine. The concepts described in the copending application have been successfully demonstrated with confirmation that a machine can be produced of smaller size and weight and lower cost, or of higher power output, by the circumferentially segmented magnetic configuration as opposed to axially segmented magnet homopolar machines as are described in Mole, Pat. No. 4,041,337, issued Aug. 9, 1977. The circumferentially segmented magnet configuration makes it more practical to use brushes for current collection and adequate magnetic fields can be provided without resort to excessively bulky field coils or the complication and expense of superconducting magnets.
There remains an interest in the art to provide machines having high power density in more simplified structural configurations, particularly insofar as the minimization of electrical current collecting brushes is concerned. Applications of particular interest include ship propulsion motors and generators and other such applications where high power density machines are required.
In the copending application, embodiments of circumferentially segmented magnet machines are disclosed that generally require a large number of current collecting brushes. Each conductor of the rotor passing through the active zone requires a brush set at each end, with each set sized to carry full winding current. The situation is not appreciably improved by using brushes contacting a plurality of rotor bars simultaneously since the currents are subdivided among the bars but not among the brushes. Therefore, multiple turn windings on the rotor cannot be used to reduce the number of brushes and maintain the same power level. Fundamentally, what is needed to develop a higher voltage is that the number of passes of conductors through the active zone and the number of effective brush sets must be increased. In machines of this general character, it is the case that machine efficiency is greatly influenced by the numbers of brushes employed because of the large friction producing surface area and the large contact drop loss at each brush-conductor interface. Designs with smaller brush area are therefore desirable to result in machines whose efficiency is more determined by torque producing conductors rather than by losses of brushes. Also with fewer brush sets, the voltage gradient along the circumference of the commutator may be reduced.
The approach taken by the present invention to solutions of the foregoing problems is to use a novel magnetic geometry in the stator structure of the circumferentially segmented magnet machine that allows the rotor to have fixed interconnections between pairs of rotor conductors or bars. This reduces the number of required brushes and their associated losses. The stator comprises a field winding in a circumferentially segmented array generally in accordance with the copending application providing active zones and null zones. The magnetic core of the stator, however is also segmented such as by providing a gap or a non-magnetic spacer between stator iron portions of adjacent pole segments. The non-magnetic region, located in a null zone, ensures the magnetic isolation of one pole segment from the next. The magnetic configuration of the machine is such that the north magnetic poles of two adjacent pole segments are located adjacent to one another, separated by a null zone. The south poles of two adjacent segments are located diametrically opposite to the two adjacent north poles. There may be any even number of pole segments in the machine. When more than two segments are used, the polarity of additional poles alternates as one proceeds circumferentially from the two adjacent north poles to the two adjacent south poles. The field windings are located in the null zones and are interconnected so as to generate the magnetic field configuration described.
Stator conductors are located in slots in the active zones, connected in circuit with the brushes, to provide mmf to compensate the rotor mmf. This minimizes undesirable voltage gradients in the active zone and assists current switching at the end of the active zone and minimizes circulating current between parallel rotor conductor circuits.
The rotor is constructed generally in accordance with the embodiments of the copending application in that there are a plurality of longitudinally running, circumferentially spaced, rotor conductor bars on its outer periphery, preferably as an air gap winding although a winding in slots may also be employed. Significantly, the rotor conductors have fixed interconnections at their ends that create a series path through a plurality of the rotor conductors that are spaced a pole distance apart. The fixed interconnections are selectively provided with current collector bars for contacting the associated brushes. Brushes need be located only between two adjacent north poles and two adjacent south poles of the stator. When the current enters one brush, it splits with half going in opposite circumferential directions to rotor bar sets in different active zones. The currents pass circumferentially through end connections to rotor bars under the adjacent pole until the opposite brush set is reached. While it is possible to arrange the conductors with their brushes at only one end of the machine, it is preferred that the arrangement be such as to provide brushes on both ends of the machine, while still minimizing the numbers of required brushes as opposed to the embodiments of the copending application. This permits at all times that the conductors which are totally within the active zones of the stator magnetic field carry current.
Breaking down the elements of the basic arrangement in accordance with the invention, one finds the following characteristics:
(a) The stator has windings and a magnetic core structure providing a sequence of active and null zones around the circumference. Some even number of pole segments are provided, with twice that number of active zones. For example, considering a four pole segment machine, each active zone may encompass about 30.degree. and each null zone about 15.degree.; a 90.degree. stator quadrant, beginning at one active zone, thus comprises 30.degree. active, 15.degree. null, 30.degree. active, and 15.degree. null zones in sequence and a single location within one active zone is displaced about 45.degree. from a single corresponding location in the next active zone. A pair of north poles are adjacent each other, with a null zone between them, and a pair of south poles are adjacent each other with a null zone between. (All angles referred to are mechanical, rather than electrical, unless the context makes clear otherwise.)
(b) The number of rotor conductors is preferably large so each bar subtends only a small angle. Normally, this means a significantly larger number of rotor conductors than active zones. For example, sixty-four conductors may be provided for a machine as referred to in (a) which has four pole segments and eight active zones; and the rotor conductors are substantially uniformly spaced around the rotor (e.g. at a given rotor position, two thirds of the conductors are in active zones.)
(c) In the stationary structure, at least one brush pair is provided for contacting rotor bars that are in the two adjacent north poles of the field and in the two adjacent south poles.
(d) The rotor conductors are interconnected into a number of series sets each comprising conductors displaced from each other substantially by the 45.degree. displacement of stator active zone locations. In a sixty-four bar example with eight active zones, eight bars spaced a pole distance may be connected in two four-bar series sets, the two sets being directly parallel by the brushes. A collector bar is provided at each interconnected pair of bars.
(e) The brush set contacts diametrically opposite positions of a series set of rotor bars while the bars of that set are located in active zones; the voltage generated therefore is that developed across the series set rather than a single bar or plurality of parallel bars.
In addition to the above, two brush sets may be used, one at each end of the machine, with the rotor bars interconnected in sets having staggered collector bars at each end. The rotor bars themselves may be disposed on the rotor in a single layer or in two layers of individually insulated bars.
Furthermore, winding conductors may be placed in parallel under the brushes so long as the parallel connected windings are within the active zone. The winding voltage may be increased by using multiple turns per pole which means that the turns that are closed under like poles do not add to the voltage of the machine so that there is an advantage to using a larger number of poles to minimize the effect of cancelled voltages.
Therefore, it can be seen that by the provision of the stator with non-magnetic spacers between adjacent pole segments and each pole segment having two active zones separated by a null zone that the rotor bars can be interconnected to achieve high power density and reduced current collection requirements.